Hitherto, a polybutadiene containing a cis-1, 4 bond of not less than 80%, which is synthesized using nickel-containing catalyst, has been mainly used as a base rubber of a rubber composition for a one-piece solid golf ball, a core of a solid golf ball (such as a two-piece golf ball or three-piece golf ball), or a solid center of a thread wound golf ball. This is because the nickel-catalyzed polybutadiene imparts excellent rebound characteristics and good durability to the golf balls.
It is also known to the art that a lanthanide-catalyzed polybutadiene, i.e. a polybutadiene synthesized using lanthanoid-containing catalyst, is used for the application of golf balls. For example, Japanese Patent No. 2678240 proposes that a rubber mixture of
85 to 15 parts by weight of a nickel- or cobalt-catalyzed polybutadiene having a cis-1, 4 content of more than 40% and a Mooney viscosity of less than 50, and
15 to 85 parts by weight of a lanthanide-catalyzed polybutadiene having a cis-1, 4 content of more than 40% and a Mooney viscosity of less than 50 can be suitably used for a golf ball. However, when the nickel- or cobalt-catalyzed polybutadiene having such a low Mooney viscosity (less than 50) and the lanthanide-catalyzed polybutadiene having such a low Mooney viscosity (less than 50) are used as base rubber for a rubber composition of golf balls, the resulting golf ball does not have sufficient rebound characteristics and durability.
A thread wound golf ball is composed of a solid center, a thread rubber layer formed on the solid center and a cover formed on the thread rubber layer, and the thread rubber layer is believed to give rebound characteristics to the golf ball. In order to obtain high launch angle and low spin amount for extending flight distance, it has recently been proposed that the thread rubber layer is made thinner and the solid center is made larger. The thin thread rubber layer does not give sufficient rebound characteristics and therefore the solid center has to have higher rebound characteristics.